Control apparatus



Aug. 30, 1949. L, CHERRY CONTROL APPARATUS Filed Oct. 6, 1945 3 Sheets$heet l FIG.I

INVENTOR. LLOYD B. CHERRY ATTORNEY.

Aug; 30, 1949. B. CHERRY CONTROL APPARATUS s Sheets-Sheet 2 Filed Oct. 6, 1945 INVENTOR. LLOYD a. CHERRY ATTORNEY.

Aug. 30, 1949- B. CHERRY CONTROL APPARATUS 5 Sheets-Sheet 3 Filed Oct. 6, 1345 R R A\ m M J M W v A P P V N T T E l U TU C A M O NO l E R W m NR 8 T o 0 I PT T D A A W T m 0 U V o M m L P A T l M Y m v T R Mm m A N C a l 5 E m D @D Q mm c E c R A\ m F v IW m0 FJO ww O MQ FJO E E E m m m T T T. R .l T '1 m 0 U m V M P F P H N W O L L L I. TU M m m M m I m N o R R E m m o w c 0 W 5 s T s s "M OT PA H H. m M H D M, WN R I m m m w m m m m Y A M H A W I! w NC, C L 4 L P l N L H H N D E m C C l U S S S c Q 0 O O u I C E R F modhJo M04230 m0 P40 mO hJO E E M m M U T T INVENTOR. LLOYD s CHERRY ATTORNEY.

Patented Aug. 30, 1949 oon'rnoi. APPARATUS Lloyd B. Cherry, Abilene, Tex., assignor, by mesne assignments, to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn, a corporation of Delaware Application October 6, 1945, Serial No. 620,833

6 Claims.

The present invention relates to electrical oscillation generators, and to electrical systems incorporating such generators.

In electrical oscillation generators of the type having a parallel resonant circuit coupled to another circuit including a so-called tickler coil' for feeding back oscillation: sustaining energyinto the parallelresonant circuit, reflected impedance from the 'circuit including the tickler coil into the other circuit tends to distort the simple relationship between frequency and tuning capacity of a single parallel resonant circuit with theresult thatthissimple relationship does not obtain for the parallel resonant circuit. While this distortion may be of no consequence in some applications of oscillation generators, it is intolerable in others. For example, when the oscillation generator is utilized in 'a sensitive follow-up or position reproducing system of the frequency discriminating type disclosed in application Serial No. 537,505, filed by R. F. Wild on May 26, 1944, this distortion causes the movements of the controlled element to be non-linear with respect to the movements of the controlling element. Such operation obviously is undesirable and cannot be tolerated in position reproducing systemsin which accuracy of position reproduction is of prime importance.

Accordingly, a general object of'the invention is to provide in electrical oscillation generators of the type referred to means'to eliminate or reduce to a negligible minimum the tendency to distortion of the simple relationship between frequency and tuning capacity of "the parallel resonant circuit. 7

A more specific object of the invention is to provide improvements in position reproducing systems resulting in the attainment of greater accuracy of position reproduction.

According to the present invention, the tendency to distortion of the simple relationship between frequency and tuning capacity of the parallel resonant circuit of an electrical oscillation generator having a tickler coil coupled to the parallel resonant tuning circuit is reduced to a negligible minimum by connecting a condenser in parallel with the"tickler coil. This condenser is so chosen as to tune the circuit including the tickler coil to a frequency which is approximately midway between the upper end'of the operating band of frequencies (over which the oscillating currents produced by the oscillation generator are adapted to be varied as the tuning condenser of the parallel resonant circuit is ad- ;illSted from one extreme value of capacity to the ,v

, 2 e other) and the lower end of the corresponding second harmonic frequency band.

It has been discovered that when the condenser connected in parallel with the tickler coil is so chosen, the relationship between frequency of oscillation of the generator and tuning capacity in the parallel resonant circuitapproaches quite closely the simple relationship'between frequency and tuning capacity of a simple parallel resonant circuit, any distortion which ,may'exist' being negligibly small. Consequently, when the principles of the present invention are embodied in position reproducing systems, for example, the movements of the controlledelement are substantially linear with respect to the corresponding movements of the controllingelement.

In an alternative embodiment of the present invention, the desired relationship between frequency of oscillation ofthe generator and the tuning capacity of the parallel resonant circuit is obtained by assigning a value to the condenser tuning the tickler coil such that the circuit including the :tickler coil is tuned to a value approximately midway between theupperend of the second or a higher harmonic frequency band *and the lower end of the next higher harmonic frequency band. -When the tuning condenser for the circuit including the tickler coil is so chosen,

the relationship between frequency of oscillation of the generator and the tuning capacity, in the parallel resonant circuit, also approaches quite closely thesimple relationship between frequency and tuning capacity of a single parallel resonant circuit. 1

One particularly advantageous application of the present invention is in connection with telautograph system s I utilized to transmit plotted data or written information from a transmitting station toone or more remotely located receiving stations In such systems accuracy of reproduction of *thetransmitted material is of great importance. *T'hisresult may be realized by the use of the present invention, as will become apparent as the description proceeds.

Accordingly, another specific object ofv the invention is to provide-improvements in telautograph systems to the end of effecting greater accuracy in reproduction of the transmitted material. v I

- The various features of novelty which characterize thisinvention are pointed out with particularity in the claims annexed to and forming a part ofthis specification. Fora better understandingof the invention, however, its advantages and specific objects obtained with its use,

reference should be had to the accompanying drawings and descriptive matter in which is illustrated and described a preferred embodiment of the invention.

In the drawings:

Fig. 1 illustrates a wiring diagram of the transmitter mechanism of a preferred embodiment of the invention;

Fig. 2 illustrates a wiring diagram of a preferred form of receiver mechanism which may be employed in conjunction with the transmitting mechanism of Fig. 1; and

Fig. 3 shows voltage diagrams illustrating the operation of the transmitting and receiving mechanism of Figs. 1 and 2.

In Fig. 1 there is illustrated the transmitter of a telautograph system embodying the principles of the present invention. The transmitter includes means for producing two separate variable frequency high frequency oscillating currents,

each corresponding in frequency to the deviation of a marking element or stylus I from two mutually perpendicular lines or coordinate axes which have been designated by the reference characters 2 and 3, respectively, and are drawn on a'plotting board or table '4. The marking element or stylus I may comprise either a pen or a pencil and for purposes of illustration will be considered to be a pencil.

Data is plotted on-the plotting board 4 with the pencil I, the latter being suspended from a so-called transmitting head indicated generally by the reference character 5. The transmitting head 5 has been shown in very diagrammatic manner in Fig. 1 inasmuch as the details of that device comprise no part of the present invention, since that device is the invention of George W. Barnes, Jr., John F. Goetz, and Walter P. Wills and is disclosed in an application Serial No. 62 0,- 82'7, filed concurrently herewith. A writing handle is preferably attached to the holder 6 for the pencil to facilitate the plotting of information 7 to be transmitted on the plotting board 3. The

pencil, when not in use, is automatically lifted from the plottingboard 4 by spring means located within the holder 8 and disclosed in detail in the Barnes, Goetz and Wills applications. A switch I is provided in the pencil holder 8 for actuation by writing pressure of the pencil on the plotting board for the purpose of actuating a solenoid at the remotely located receiver shown in Fig. 2 so that the marking element at the receiver is adapted to engage the recording chart only when the transmitter pencil is in contact with the plottin board. A knurled nut 8 at the top of the pencil holder is provided to permit the latter to be turned, whereby plotting can be done from any side of the plotting hoard. After the pencil holder has been adjusted to a desired position, the knurled nut should be tightened.

The transmitting head includes certain of the electrical components of the transmitting mechanism and in particular houses variable electrical condensers 9 and I8, and also includes suitable cams and gimbal joints so that the condenser 9 is arranged to be varied in capacity in accordance "with the movements of the pencil I along one coordinate, for example, along the coordinate 2, while the condenser I0 is arranged to be varied in capacity in accordance with the movements of pencil I in the other coordinate.

The cams referred to serve a compensating purpose required because the angle of the deflection of the pencil holder 6 is not directly proportional to the linear movement of the pencil 1 along the plotting board. The transmitter head also includes two electrical oscillation circuits II and I2, respectively, associated with condensers 9 and I0 Oscillation circuits II and I2 may be and are shown as being identical to each other, and therefore, only oscillation circuit II will be described. Components of the oscillation circuit I2 corresponding to those of the oscillation circuit II have been indicated by the same reference numeral with a prime figure added.

Oscillation circuit I I comprises an electron coupled oscillator and includes a pentode tube I 3 which may be of the commercially available type 6SJ7. Tube I3 includes an anode, a suppressor grid, a screen grid, a control grid, a cathode and a heater filament. The heater filament is connected to and receives energy from the secondary winding I 4 of a combination stepup-stepdown transformer I5 having a line voltage primary winding I6 and high voltage secondary'wind- -a suitable fuse. The fuse-is provided to protect the apparatus from damage due to excessive currents resulting from failure of a component of the apparatus. -A lamp 24' connected across the terminals of the transformer secondary winding I4 is employed to provide a visual indication that the switch 24 is adjusted to its on position and that the apparatus is powered for operation.

The control grid of tube I3 is connected through a parallel connected resistance 25 and condenser 26 to one terminal of a parallel circuit 21, the other terminal of which is connected to ground G. The parallel circuit-21 includes in one branch an inductance coil 28 and includes condenser 9 in an other branch. The other branches of the parallel circuit respectively include condensers 29, 30, and 3|. Condenser 3I is shown as adjustable in capacity and is provided for the purpose of providing a fine adjustment for the zero setting of the pencil I along the coordinate 2. Preferably the condenser 3I is provided with a suitable knob or kerf to facilitate its adjustment. Condenser 29 preferably is of the type having a negative temperature coeificient of capacitance and is provided to compensate for the effect of ambient temperature variations upon the frequency of the oscillating current generated by oscillator II.

The cathode of tube i3 is connected to ground G through an inductance coil 32 which is disposed in inductive relation to the coil 28 and serves the purpose of feeding back energy from the output circuit of the tube I3 to the circuit of the control grid for maintaining the circuit H in an oscillating state.

Due to the coupling between the tuned circuit including coil 28 and the circuit including coil 32, which latter circuit, excluding condenser 32a, is also a tuned circuit because of distributed, wiring and tube capacities, the impedance of the circuit including coil "32 is reflected back into the circuit including coil 28. This efiect tends to terminal of resistor 44 is connected tion of the transmitted material is of prime importance. 'While this distortion can be kept small by loosely coupling coils 28 and 32, it can be reduced still further to a negligible minimum by the use of a condenser 32a connected in parallel with inductance coil 32. Condenser 32a is so chosen as to tune the circuit including inductance coil 32 to a frequency which is approximately midway between the upper end of the operating band of frequencies (over which the oscillating currents produced by oscillator I I are adapted to be varied as condenser 9 is rotated from oneextremeposition to the other) and the lower end of the corresponding second harmonic band. It has been discovered that when condenser 32a is so chosen, the movements of the receiver marking element are substantially linear with respect to the corresponding movements of the transmitter stylus I over the entire range of operation.

Alternatively, the condenser 32a may be so chosen as to tune the circuit including the inductance coil 32 to a frequency which is approximately midway between the upper end of the second or a-higher harmonic frequency band and the lower end of the next higher harmonic frequency band. In sucha modified arrangement the movements of the receiver marking element will also be substantially linear with respect to the corresponding movements of w the transmitter stylus I over the operating range;

As shown, the screen grids of tubes I3 and I3 are connected through respective resistors 33 and 31 to the positive terminal 34 of a filter indicated generally by the reference character 35 and having its input terminals connected to the output terminals of a full wave rectifier designated by the character 36. The suppressor grids of tubes I3-and I3 are connected directly to ground G. t

The oscillating circuit of oscillator II includes the control grid circuit of which the parallel circuit 21, including the variable condenser 9, forms a part and also includes the screen grid circuit, which maybe traced from the positive terminal 34 through resistance 33, the screen grid of tube I3, the cathode thereof, and inductance coil 32 to ground G, which, as shown, constitutes the negative output terminal of the filter 35.

The screen grid and control grid circuitsare inductively coupled by the inductance coils 28 and 32 and provide for high frequency operation through a range varying from approximately 390 to 4'70 kilocycles, depending upon the position along the coordinate 2 of the pencil I. Similarly, H

the inductance coils '28 and 32' of oscillation circuit I2 provide for high frequency operation through a frequency range varying from approximately 390 to 470 'kilocycles, depending upon the position of thetransmitter pencil I: along the coordinate 3. Accordingly. the oscillation circuits II and I2 each'produce a high frequency oscillating current having a frequency within the range of 390 to 4'10 kilocycles, depending upon the position to which the pencil l of'the trans- 2 and 3.

Energizing current is supplied the anode circuit of the oscillation circuits I I and I2 from the rectifier 38 and filter 35 through a circuit which may be traced from the output terminal 34 of filter 35 through a conductor 42 to the junction of a pair of resistors 43' and 44. The other terminal of resistor 43 is'connected to the anode of tube I3 of oscillation circuit II, while the other of tube I3 of oscillation circuit I2. The cathodes of tubes I3 and I3 are connected through their respectively associated feedback coils 32 and 32' to ground G and thereby to the negative output termina1 of filter 35. It is noted that ground G is connected by a resistor 45 to the junction of the transformer secondary windings I8 and I9 whereby the potential of the junction point of windings I8 and I9 is maintained suitably negative with respect to ground potential.

In order that both of the high frequency oscillating signals generated by the oscillation circuits I I and I2 may be amplified by a, common amplifier and both amplified quantities may be conducted. over the same transmission lines to the remotely located receiving station, the oscillating currents produced by the oscillation circuits I I and I2 are alternately interrupted at a suitable frequency, and in particular, at the frequency of the voltage supplied by source 23. To this end there are provided keying and amplifying tubes 38 and 39. Tubes 38 and 39 also serve as buffer tubes to increase the stability of the respectively associated oscillation circuits II and I2 by reducing fluctuations in the oscillation circuit loads.

The use of tubes 38 and 3 9 for alternately interrupting the operation of the oscillators I I and I2 and for serving the buffer purpose noted com- -prises no part of the present invention but is disclosed and claimed in application Serial No. 620,829, filed concurrently herewith by Rudolf F. Wild, and Fred J. Curran, which has issued as Patent 2,457,790. The connections of tubes 38 and 39 in the circuit and their manner of operation will now be described.

As shown, each tube 38 and 39 includes anode, suppressor grid, screen grid, control grid, cath- 'ode, and heater filament elements. Energizing current is supplied each of the heater filament elements from the low voltage secondary winding I4.

The output circuit of the oscillation circuit II is coupled by means of a condenser 49 to the screen grid of tube 38 for amplifying and transmission purposes, while the output circuit of oscillation circuit I2 is coupled by means of a con- :denser M to the screen gridof tube 39 for the same purposes.

Energizing current is supplied the anode circuits ofthe tubes 38 and 39 from the rectifier 3B and filter 35 through a circuit which may be traced from the positive output terminal 34 of the filter to the junction of resistors 33'and 31. The other terminal of resistor31 is connected to the anode of tube 38, while the other terminal of resistor 33 is connected to the anode of tube 39. The cathodes of tubes 38' and 39 are connected together and to the junction of transformer secondary windings I8 and I9. Since the potential of the last mentioned junction point is negative with respect to ground by an amount determined by the magnitude of resistor 45 and the magnitude impressed on the tubes 38 and 39 is of corremitter is moved relatively, to the coordinates 65' spondingly greater magnitude than that impressed on the anode circuits of tubes I3 and I3.

LResistor 45, accordingly, is so chosen as to provide the proper operating anode voltages for tubes j38'and 39.

The suppressor grids of tubes 38 and 39, as shown,,ar e directly connected to thecathodes of :said tubes.

to the anode filter 35 through a circuit which may be traced from the filter output terminal "34 to the junction point of resistors 45' and 49. The other terminals of resistors 85' and 46 are connected to respective screen grids of tubes 38 and 39.

Separate alternating voltages 180 out of phase with each other are impressed on the control grids of tubes 38 and 39 from the transformer secondary windings I9 and I 8, respectively. Specifically, the cathodes of tubes 38 and 39, as previously noted, are connected together and to the junction point of windings I8 and I9. The other terminal of winding I9 is connected through a resistor 43 to the control grid of tube 38; while the other terminal of winding I8 is connected through a resistor 49 to the control grid of tube 39. The alternating voltages thus impressed on the control grids of tubes 38 and. 39 are of the proper magnitude to render the tubes 38 and 39 non-conductive during alternate half cycles of the alternating voltage supplied from the alternating current source 23. V V

In order to periodically interrupt the oscillating signals generated by the oscillation circuits I I and I2 at the frequency of the alternating voltage supplied from source 23, the screen grid of tube I3 is connected to thejunction of resistor 33 and the anode of tube 39 and the screen grid of tube I3 is connected to the junction of resistor 31 and the anode of tube 38. When tubes 38 and 39 are rendered conductive, the potential of their respectively associated anodes decreases with respect to ground potential. The reduced anode voltages are sufficiently low when applied to the screen grids of tubes I3 and I3 to cause the latter tubes to be rendered non-conductive. Hence, tube I3 is arranged to be rendered non-conductive during the half cycle that tube 39 is conductive. During that half cycle tube 38 is non-conductive, and in consequence, tube I3 will be conductive and an oscillating signal will be produced by oscillation circuit I2. In the next half cycle tube 38 will be conductive and the oscillatingsignal produced by the oscillation circuit I2 will be interrupted. During this latter halfcycle the tube 39 will be non-conductive, and as a result the oscillation circuit II will be operative to produce an oscillating signal, The fluctuating voltages 50 created and impressed on the screen grids of oscillator tubes I3 and I3 change rapidly from a value at which the oscillators are permitted to oscillate to a value at which oscillation is interrupted, and consequently, the interruption and initiation of the state of oscillation of tubes I3 and I 3 is clean and sharp.

Specifically, assume that the voltage applied to the control grid of tube 38 is traversing its positive half cycle while the voltage applied to the control grid of tube 39 is traversing its negative half cycle. The amplitude of the voltageapplied to the control grid of tube: 39 is suificient to.

quickly block or cut off conduction through tube as. Therefore, this tube is effectively eliminated from the circuit and the screen grid of oscillator tube I3 receives it full operating voltage from the output terminal 34 of the filter 35. It is important to note that during operation of the oscillation circuit II, the operating voltages applied to the electrodes of tubes I3 are constant for all practical purposes, and consequently, no frequency modulations occur due to periodically varying values of operation voltages.

During the half cycle under consideration tube 38 is conducting and the steady D. 0. component of its anode current efiects a sharp decrease in the voltage of its anode, which voltage is also the voltage applied to the screen grid of oscillator tube I 3, to such a value that conduction through the oscillator tube I3 is suddenly cutoff. It is noted that cut oil of tube I3 is readily accomplished because the anode voltage of tube 38 may become zero or even go slightly negative with respect to the potential of the cathode of tube I3 and still provide an operating voltage for the anode of tube 38 of amount sufficient to maintain conduction through tube 38. This operation is made possible because of the negative voltage applied to the tube 38 cathode by virtue of the latters connection to the junction oftransformer secondary windings I8 and I9, which, as previously indicated, is negative with respect to ground potential.

In the half cycle under consideration the tube 38 is utilized as a buffer for the oscillation circuit II and also serves to amplify the oscillations produced by that oscillation circuit. To this end, the anode of tube I3 is connected by condenser 40 to the-screen grid of tube 38. It is noted that the circuit constants are not critical as long-as the most positive voltage which the anode of tube 38 may assume during this interval determined by the direct current and radio frequency voltage drop across the anode resistor 31 is sufiicient to prevent oscillation of the oscillation circuit I2;

For the next half cycle of the voltage supplied by source '23, the reverse of the above conditions exists. That is to say, tubes 38 and I3 are cut on" while tubes 39 and I3 are rendered con- I ductive. Hence, during this half cycle the oscillation circuit I2 produces an oscillating signal and this oscillating signal is amplified by tube 39 which also serves a buffer purpose, To this end, the anode of tube I 3' is connected through condenser II to the screen grid of tube 39.

From the foregoing description it is evident that the interruptions of the oscillating signals produced bythe'oscillation circuits II and I2 are timed so that one'oscillation circuit I I generates an oscillating signal only when the oscillation circuit I 2 is cut off and vice versa. Moreover, the

interruption of the oscillating signals from the oscillation circuits II and I2 occurs at the frequency of the alternating voltage supplied from the source 23. In other words, during one half .cycle of the alternating voltage supplied by source 23, the oscillation circuit II is operative to generate an oscillating signal and that oscillation signal is amplified by the tube 38. During the next alternate half cycle the oscillation circuit I2 is operative to generate an oscillating signal and that signal is amplified by the tube 39. Therefore, the tubes 38 or 39 which generate a voltage for interrupting one of the oscillating signals are also used to amplify the output of the other oscillation circuit during the cut off period of the first oscillation circuit. The second tube 38 or 39 which generates a voltage for interrupting the other oscillation circuit performs a similar amplifying function for the first oscillating circuit. This amplification stage comprising tubes 38 and 39, as previously noted. also serves to increase the stability of the oscillation of oscillation circuits I l and I2 by reducing fluctuations in the oscillation circuit loads.

In order to further amplify the oscillating signals generated by the oscillation circuits II and I2 sufficiently for transmission of those oscillat ng signals to the remotely located receiving station or stations, there is provided an additional amplifier indicated generally by the reference charactor 54. This amplifier 54 also performs the additional function of matching the impedance of the output circuit of the transmitter to the impedance of the receiver or receivers to obtain maximum power transference.-

As shown, the transmitter amplifier 54 includes a tube 55 which may desirably be of the commer- "cially available type BAC'! and includes anode, suppressor grid, screen grid, control grid, cathode, and heater filament elements. Anode voltage is supplied tube 55 from the rectifier 36 and filter 35 through a circuit which may be traced from the positive output terminal 34 of the filter through a resistor 56 to the anode of the tube 55, the cathode thereof, and through a biasing resistor 51, shunted by a condenser 58, to ground G and thereby to the negative output terminal of the filter.

Energizing voltage is supplied to the screen grid of tube 55 through a circuit which may be traced from the positive output terminal 34 of the filter through a resistor 59 to the screen grid, the oathode and the parallel connected resistor 51 and condenser 58 to the negative and grounded output terminal of the filter. The screen grid is also connected by a condenser 60 to ground. The suppressor grid is connected directly to the cathode of tube 55.

The output circuits of the keying and amplifying tubes 38 and 39 are coupled in parallel to the input circuit of the tube 55 so that the mixing of the two oscillating signals derived from the oscillators H and I2 takes place in this circuit. Specifically, the anodes of both tubes 38 and 39 are connected by respective condensers 6| and 62 to the control grid of tube 55, which, in turn, is connected by resistor 62 to ground G, and the cathodes .of tubes 38 and 39 are connected through resistor 45 to ground G and through the parallel connected resistor 51 and condenser 58 to the cathode of tube 55. Tube 55, therefore, operates to amplify the oscillating signals in the output circuits of both tubes 38.and 39. The amplified quantity derived from tube 55 is resistance coupled by means of a condenser 63 and a resistor 65 to the input circuit of a tube 65 which isprovided for the purpose of matching the impedance of the output circuit of transmitter amplifier 54 to the characteristic impedance of a transmission line connecting thetransmitter to the remotely located receiver.

Tube 65 may be of the commercially available type (W6 and includes an anode, a screen grid, a control grid, a cathode, a heater filament, and also a pairof beam forming plates. Energizing voltage is supplied to the anode circuit from the rectifier 36 and filter 35 through a circuit which may be traced from the positive output terminal 34 to the anode of tube 65, the cathode thereof, and through an output resistor 66 to the negative and grounded terminal .of the filter. Energizing voltage is supplied the screen grid of tube 65 through the samev circuit. The-beam forming plates, as shown, are directly connected to the cathode. The control grid of tube 65, as shown, is directly connected-to the junction of condenser 63 and resistor 64 and thereby is coupled to the output circuit of tube 55.

While the rectifier 36, filter 35, and transmitter amplifier 54' have been shown in Fig. 1 as being located closely adjacent the plotting board 4, it will be understood that, if desired, these components may be located at a'distan'ce with respect to the plotting board and also with respect to the oscillation circuits II and I2. For example, in some applications it-may be desired to locate the" .10 plotting board 4 and the oscillation circuits H and I2 in one room and to have the rectifier 36,

. filter 35, the transmitter amplifier 54, and the switching mechanism to which reference has previously been made, in another room to the end that the transmitting mechanism available for manipulation by an operator may be made less cumbersome and bulky. This feature is also advantageous when the transmissionsystem of the present invention is utilizedon shipboard in which the space available is at a premium and would preclude the locationof all of the transmitting equipment closely adjacent the plotting board t and overhead transmitter unit 5. In a practical operating embodiment of the present invention, a shielded cable 25 feet long is provided between the plotting board and the circuit components including the rectifier 36,fi1ter 35, amplifier 54, and the switching mechanism.

Tube 65 is connected to operate as a cathode follower, that is to say, the output signal which is transmitted, to the receiver is derived from the resistor 66 connected in the cathode circuit of the tube. Resistor 66 is so chosen as to have a characteristic impedance approximating that of the transmission line L connecting the transmitter to the receiver. As shown, the terminal of resistor 66, which is connected to the cathode .of tube 65, is coupled by a condenser 61 to one terminal of the transmission line-'L, while the other and grounded terminal of the resistor 66- is connected to another terminal of the transmission line- At the transmitter amplifier54 there is also provided a relay 68, one terminal of the operating coil of which is connected to the ungrounded side of transformer secondary winding 15 and the other terminal of which is arranged to be connected to the grounded sidev of winding I4 upon closure ofthe switch 1 attached to the writing pencil or stylus I. To this ,end, one terminal of the switch, 1, as shown, is connected directly to ground G. When the switch I is closed, as upon movement of the stylus l into engagement with the plotting board 4, the relay 68 is energized and closes an associated switch 69to energize. a sole noid 19 in the receiver and attached to the marking element or recording pen mechanism for moving the latter into, engagement with the recording chart. When the switch I is open and the relay 68 is deenergized, the switch 69 is also open, and the pen at the receiver is moved out of engagement with the recording chart by the action of gravity, the solenoid 10 then being deenergized.

As has been previouslymentioned, the apparatus located at the remote station, where the transmitted writing or other data is to be reproduced, is arranged to translate the adjustable frequencies of theoscillating signals-generated by the oscillation circuits II and I2 into two coordinates of pen position. More specifically, the receiving apparatus-operates to move the recording pen in a vertical direction in accordance with the variations in frequency of the oscillation signal generated by'the oscillation circuit II, and operates to move the recording pen in a horizontal direction in accordance with the frequency variations of the oscillation signal generated by the oscillation circuit l2. In this manner the changes in position of the pencil or stylus along the plotting board 4 at the transmitter may be exactly reproduced at the receiving apparatus.

By reference to Fig. 2, it will be noted that two independent receiving circuits are provided at each receiver. One'of the receiving circuits controls the movement oi the, recording pen in one 11 coordinate, for examplefthe horizontal coordinate, and the other receiving circuit controls the recording pen in the vertical coordinate. The oscillating signals derived from each of the oscillation circuits II and I2 and transmitted to the receiver over the transmission line L are impressed on both of the input circuits of the two receiving circuits. The receiver mechanism is so arranged that one of the receiving circuits is responsive only to the oscillating signal generated by the oscillation circuit 1 t, and the-second receiving circuit is responsive only to the oscillating signal generated by theother oscillation circuit l2' to control the energization of individually associated reversible motors, which in turn are linked by suitable mechanical means to the recording pen to position the latter in the two ordinates along the recording chart. The operation of the two receiving circuits is the same and the following explanation applies to both circuits. One of the receiving circuits has been generally indicated by the reference character H and the other by the reference character H. The elements of the receiving circuit H corresponding to those of the receivingcircuit H have been indicated bythe same reference numerals with a prime figureadded.

The receiver" includes two stages of amplification, the first ofwhich utilizes an amplifying tube 72 of the commercially available type 6A0? and the second of which utilizes an amplifying tube 13 of the type 6V6. In the second stage of amplificationthe-two oscillating signals from the transmitter are separated and one of them is rejected. In other words the second stage of amplification responds only to the oscillating signal generated by one of the oscillation circuits l l or 2 and, for purposes of illustration, will be assumed to respond only to the oscillating signal generated by the oscillation circuit 1 I. Rejection of the oscillating signal from the oscillation circuit I2 is accomplished in a manner described in detail hereinafter, and generally speaking, involves the applicationofan alternating voltage derived from source 23 to periodically interrupt the conduction of tube 13.: The oscillating signal after rejection ofthe undesired portion consists of a radio frequencywave which is periodically interrupted at the frequency ofthe voltage supplied by the source 23 and has been illustrated sche-v matically in graphs (b) and (bb) of Fig. 3 which show the oscillating signal frequency as being higher and lower, respectively, than a frequency value at'which the receiving pen is held motionless along'the vertical coordinate.

Tube 12 as shown, includes anode, suppressor grid, screen grid, control grid, cathode, and heater filament elements. Energizing current. is supplied the heater filament from the low voltage secndarr winding H. of a transformer '15. having. a line voltage. secondary windin wh c is 0011- nec ed bycond ctdrs Band t9. tot al ern ti volta e sourc 23. To. e d he conductors .8. and 1.9 are includedin the shielded ca L connecting the transmitter. tothe receiver. The. ran former 15. als mclndes. hi h volta e,- secondary windings and 81, each of which is provided with a center tap.

Energizing voltage is supplied to. the anode, of

tube 112 from, a fullxwaverectifier indicated generally by thev reference numeral 82 and a filter indicated at. 8 .3.; The rectifierifiz energized ythe transformer secondary: 8ft provides a voltage at the output. terminal 84: of the filter 83. which is suitably. positive with respect. to. the:

' minal of the filter is connected. As shown, the

positive terminal 84 is connected through a resister 85 to the anode of tube '12, and the cathode is connected througha parallel connected resistor 85 and condenser 8 to ground G. Energizing voltage is supplied to the screen grid of tube 12 from the positive filter output terminal 84 through a circuit, including a resistor 88, to the screen grid and from the cathode through the parallel connected elements 86" and 81 to ground G. The screen grid is also connected to ground G by a condenser 38a, as shown.

As may be seen by reference to Figs. 1 and 2, the output terminal of resistor 66 of the transmitter amplifier 54, which is connected to the cathode of tube 65, is connected by condenser 61 and the shielded transmission line L to the control grid of tube 12. The other and grounded output terminal of the transmitter amplifier 54 is connected through the transmission line L to the parallel connected resistor 86 and condenser 81 and thereby to the cathode of tube 12.

The output circuit of tube 12 is coupled by a condenser- 89 and a resistor 90 to the input circuit of tube 13. Tube 13 includes an anode, a screen grid, a control grid, a cathode, a heater filament, and beam forming plates. Energizing current is supplied to the heater filament from the low voltage transformer secondary winding 14.

Anode voltage is supplied to tube 13 through a circuit path which may be traced from the positive output terminal 84 of the filter 83 through a parallel network including in one branch the primary winding of a transformer 96 to the anode of tube 13, the cathode thereof, and a parallel connected resistor 92 and condenser 93 to the grounded and negative terminal of the filter. The other branches of the parallel network respectively include a resistor 9| and a condenser 94. The transformer 96 comprises a part of a frequency discriminator indicated generally by the reference numeral 9:! and includes a split secondary winding in addition to the primary winding 95.

The beam forming plates of tube 13 are directly connected to the cathode, and the control grid is coupled to the output circuit of tube 12 by virtue of its connection to the junction of condenser 89 and resistor 98.

Alternating voltage is impressed on the screen grid of tube 13, for periodically rendering it nonconductive, from one half'of: the center tapped transformer secondary windin 80 through a circuit which may be traced from lower terminal of the winding 89, as seen in the drawing, through a resistor 98 to the screen grid of tube I3. and through the parallel connected elements 92 and 93 to ground and-therebyto. the center tap on winding 80. The screen grid of tube 13 is also connected directly to ground G by a condenser 59. The magnitude of-the alternating voltage impressed on'the screen grid of'tube 73 is so. chosen as to be sufiicient to cause the tube 13 to be rendered non-conductive orcut off during those half cycles of the voltage of source 23 in which the screen grid is driven in the negative. direction. Furthermore, the phase of this alternating volt age is suchv that the tube 131s operative to amplify the oscillating signals generated by the oscillation circuit H and transmitted to the receiver 71 only during those alternate half'cycles. of the supply line voltage in which the oscillation circuit If is operative to gene ate an osc llatin i nal- It is noted that. thecntirev transmis ions receives energizing current from the single source 23 of alternating current. This source, as described, is located at the transmitter amplifier unit and the alternating current utilized for energizing the receiving unit is transmitted from the source 23 by way of the transmitting cable L. This is done to insure synchronism of the keying action of the transmitter and receiver units and also provides the advantage of permitting the installation and use of receivers at locations where there is not available alternating power of the same frequency and phase as that delivered by source 23.

The frequency discriminator 91, previously referred to, includes in addition to the transformer 96 a pair of diode rectifiers I and I0 I, which desirably may be contained within a single envelope, generally designated at I02. One half of the split secondary winding of the transformer 96 has been designated by the numeral I03 and the other half by the numeral I04. The center tap of the split secondary winding comprising the junction of winding sections I03 and I04 is connected through a blocking condenser I05 to the anode circuit of tube 13, and more specifically, to the terminal of the primary winding 95 which is connected to the anode of tube 13. The center tap of the split secondary winding 'is also connected to the point of engagement of a pair of resistors I06 and I01.

The resistors I06 and I01 are shunted by'a resistor I08 which is provided with an adjustable tap I09. The usable output voltage from the frequency discriminator is obtained across the resistor I08 which, as shown, is shunted by a condenser IIO.

The diode rectifiers I00 and I'0I may be contained within a single tube such as the commercially available type 6H6. Each diode includes anode, cathode, and heater filament elements. The heater filament elements are connected in series with each other and receive energizin current from the transformer secondary winding 10. The cathodes of the diodes I00 and IOI are connected through resistors I06 and I01, respectively, to the junction of the transformer secondary windings I03 and I04. The other terminal of the winding I03 is connected to the anode of diode I00, while the other terminal of the winding I04 is connected to the anode of the diode l0l.

For tuning the secondary winding of the transformer 96 to the frequency of the oscillating signals impressed thereon from the output circuit of tube 13, a variable condenser III is provided. This variable condenser is arranged to be automatically adjusted in capacity as required to maintain the secondary. winding of the transformer 06 tuned to the frequency of the applied oscillatin signals. A variable condenser H2 and a variable inductance I I3 are connected in shunt with the secondary winding of the transformer 96 for a purpose which will be later explained.

The blocking condenser I05 and the condenser IIO are so selected as to present low impedance to the radio frequency oscillating currents flowin through them. The condenser 94 and the transformer primary winding 95 connected in parallel therewith are so selected as to provide high impedance in order to produce a large output signal from the discriminator. Preferably the primary winding 95 is tuned to a value whichis outside the range of frequency variation of the oscillating signals generated by the oscillation circuit II at the transmitter and impressed on the receiver circuits. By way of example it is noted that satisfactory operation has been obtained when the primary winding is tuned to a frequency of 325 kilocycles. The tuning of the primary winding 95 to this value which is below the operating range of frequency variation of the-oscillating signals conveyed by the transmission line L to the receiver 1I' comprises no part of the present invention and is disclosed and claimed in an application, Ser. No. 620,831, filed concurrently herewith by R. F. Wild, now Patent No. 2,473,401.

The frequency discriminator 91 together with the diodes I00 and IOI is provided for the purpose of deriving a voltage having an alternating component of one phase or of opposite phase and of the same frequency as the voltage supplied by source 23 accordingly as the frequency of the oscillatingsignal impressed thereon from the output circuit of tube 13 is higher or lower than the frequency value to which the split secondary winding of transformer 96 is tuned. The details of the frequency discriminator constitute no part of the present invention. Its manner of operation and'arrangement is disclosed in a prior application, Ser. No. 537,505 filed May 26,1944 by R. F. Wild, and therefore, need be only briefly described herein.

' It is believed sufficient to note that if the frequency of the applied oscillating signal is the value to which the secondary winding of the intermediate frequency transformer 96 is tuned, the outputs of the diode rectifiers I00 and IOI cancel each other and zero voltage appears across the output resistor I08 of the frequency discriminator circuit 91. If the frequency of the applied oscillating signal is a value other than that to which the secondary winding of the transformer 96 is tuned, the phase relations of the voltages applied to thediode rectifiers I00 and I0! are such that their outputs do not cancel and a direct current voltage appears across the output resistor I08. 'I-he polarity of this voltage dependsupon whether the frequency of the applied oscillating signal is above or below. the value to which the secondary winding of the discriminator is tuned. The magnitude of the voltage depends upon the extent of departure of the frequency of the applied oscillating signal from the value to which the secondary winding is tuned.

Recalling now that the oscillating signal whic isapplied to the frequency discriminator 91 is an intermittent radio frequency wave as shown in graphs (b) and (lab) of Fig. 3, it will be seen that the voltage appearing across the ouput resistor I08 of the discriminator will be an intermittent or pulsating direct current voltage whose polarity and magnitude are as stated above. Furthermore, since the applied oscillating signal is periodically interrupted at the frequency of the alternating voltage supplied by source 23, the pulsations of direct current voltage produced acrossresistor I08 have the same frequency as the voltage supplied by source 23. Graphs (c) and (cc) of Fig. 3 illustrate the voltage output produced across the resistor I06 for the respective cases of high and low frequency of the applied oscillating signal.

The pulsating voltage produced across the resistor I08 may be considered as comprising two components, (1) a steady D. 0. component and (2) an A. C. component. The A. C. component obtained when the-frequency of the applied oscillating signal is higher than the value to which the secondar winding of the frequency discriminator is tuned is shown in graph (d) of Fig. 3

Whilegraph (dd) of Fig. 3 illustrates the A. C.

component obtained when the frequency tof athe aapplied oscillating signal is Flower than the wine :to which the secondary winding 10f ithB :fneguenoy discriminator is tuned.

The variableoondenser J12 -.eonnected .in;para1- lel with the discriminator-secondary winding sis n trimmer-condenser andsenves :to provide for zero adjustments of the record n fpen along the woratical coordinate. To this end the condenser H2 is provided with a suitable knohror akerf vfor facilitating .its addustment.

In order to provide for span adjustment or, in other words, the extent -of variation 10f --.eondenser -I'I:I required to traverse the entire iueguency range of operation, the variable inductance I-iI3-is provided. Inductance A13 is preterablyprouided withan adjustable ironoore. .A -suitabIe Jmob or :kerf is preferably provided on the inductance M3 to facilitate its adjustment.

The alternating component .of the pulsating ivoltage produced across resistance -l8l.is amplified by a stage :of voltage amplification including a tube H4, which stage of amplification aincorporates a gain control for zmanuall wjusting .the sensitivity of the receiving .unit. This gain :control comprises the provision of sliderrcontaot 4119 in association with the resistor I418 do! tappin oil and impressing on the input .circuit of .tube

I It a variable portion -of the ivoltage produced across the resistor I08. Tube :IM'JmayherQf the zcommercially available type 307 and emcludes :anode, suppressor grid, screen grid, control i rid, cathode, and :heater filament elements. Energizing current is supplied the heater filament from the transformer secondary winding .14. Anode voltage is supplied tube 4.4.4 ,from the rectifier '82 and the filter :83 and to this find the positive terminal '84 f the filter 83 is iconneeted through a resistor =I=I 5 to :the anode aOf ituhe Mi l, and the cathode of the latter is connected througharesistor H6 .to theigroundedianfi negative terminal of the filter. Ener izing wattage is supplied the screen .grid through a circuit which may be traced from the ip'ositive iteizminal .8 4 through a resistor l! tozthe screenggrid, athe cathode, and resistor :I It :to the :grounded :and :negative terminalbfithe filter. ai-condenser fim is connected between the :screengridmf' tube tm and ground, as shown.

The output circuit of tube 'I f4 --is -coupled 'by =a condenser H9 and a resistor H211 to -the input circuit of a motor drive stage indicated generally by the reference character *IZI. Motor drive stage I2! is shown as' including'triodes designated by the numerals 'I22;and'I23,"respedtiuly, but it is preferred to utilizea number of :such triodesconnected in parallel with eachotherinstead of single'triodes. "For examplefin'agaracti- .cal operating embodimentpof '.theiin\ention,;th1:ee such triodes I22 and U3 are connecteiilin panal- 'lel, thusinsuring the derivation item the ,mdtor controlled by the tubesof sulficient 'torque .to actuate .the recording .pen.

As shown, the tubes in and 1.23 ,eachjnclude anode, control grid, cathode, ,and .heater .filamerit elements. Energizing .voltage "is supplied .the'heater filament elements inserieaffromtthe transformer secondary .winding :14. Anode uoltage is supplied -tubes I22 .and .I23 from [the transformer secondary .winding-8.l .and 1&0 end oneend terminal of windingtil Ii-isconnected .to the: anode ofttube. I-2 2.-a.nd;the othenend termi- :nalof that winding is connected gtoetheeanodeflof tube I23. .Thecathodesrof tubesztH-aanddfimre connected together and through a biasin -re- .sistor I24 toone terminal of the control winding .-I= 2-.5 .of 'a reversible two-phase induction motor 12,6. 'In .addition to the winding I25, the .motor 4.12.6 includes a second winding -I.2I. .Acondenser .128 is connected in :parallel with winding I25 qandethe other terminal of the latter .is connected @torthe oenter tap of the transformer secondary windin ;8I. Accordingly, energizing :current is delivered to theemotor winding -I25 from the :transformer secondary winding .8 I under control iof .the .triodes i 22 and I23. Winding IZIof the motor I26 is connected through a condenser I29 .andconductors I8 and I9 torthe sourceof altermating current 23. w

Thus-the output currentofthe tubes 2I22 and 4231s utilized to deliverenergy to Winding I25 of 42116111101301 I26, and the power winding ;I2'I, of .that :motor is connected to the alternating cursrent :power supply through condenser I 28. The @motor is actuated for rotation when analternat- =ingqcurrent signalof the frequency of the voltage :supplied by sourc 23 is impressed on the input =circuits of tubes I22.and I23. For accomplishzingthis-result-the control grids of triodes I22and 4-23 are connected together and to the junction .0f-. c ondenser IIS and resistor I20.

Referring to graphs (d) and dd} of Figure 3, it will be seen that the A. C. componentof the discr m nator output voltage reverses in,phase when -.the :trequency of the applied oscillating signal .ohanges from below to above that to which the .frequencydiscriminator is tuned. change in .phase when applied to the control grids of-triodes I22 and I23 causes a decrease in thecon- .duotivity of one triode I22 or I23 and acorre- .sponding-increase-in the conductivity. of theother ztriode. In consequence, energizing currentris dc- Jiveredgto the control-winding I125 of-themotorL-Zfi go which gleads or lags by approximately 90 the .uoltage of source 23 depending uponwhichtriode #22 01 I23 has had its conduction -increased. .Stated di1ferently, this change in phase :pf .the svoltage derived from the frequency discriminator pauses-reversal of the direction of=r0tation of the :reversiblemotor I26.

wThedetails of this motor drive circuit comprise rno part =-of :the present inventionsince the motor drive =BiITOuitis the invention of Wa1terP. Wills .50 -and is :disclosed in a copending-application, Ser.

-No. 1A21, 17j3,' filed December 1, 11941, which issued :asilatent No. 2,423,540 on July 8, 1947, and hence,

requires no further descriptionherein.

*ilfhe-shaftzof motor I26 is geared inanysuit- ;ab1e-manner,'not shown in order tux-avoid comiplicaticn ofz the drawing; to the variable condenser :HiI connected in the frequency discriminator. iifhe shaft of motor I26 is also :mechanicalli tcoupled to the recording pen of :the--receiver:and

e radjusts-both-the condenser l H and the recording pen-.iintil :the :secondarywinding of-thefrequency gliscriminator is tuned to the frequency of the eappliedaoscillating signal. Inasmuch as the out- -putofthe; frequency discriminator is- -zero when it @ismunedto thefrequenc of the applied oscillateingsignal, thejmotor then stops; and isimaintained estationary until thefrequency of the applied os- .ciliating signal-again changes.

tInzFig. 2' a: portion of: the receiving: instrument zmmntainingthe recording chart isb ioken eawayto sshow a pen carriage I30, located beneath: the chart -,I3.8.and having azpeni I.3l mountedthereon. {The I. icy-arranged to be,- lifted by the solenoid :10 dip .into; engagement with the chart. I'38,-ga;suitable e writing; latenoe lass or-etber suitable-transparw re ent material being arranged-closely"- adjacent the top-er the chart to provide backing for t e fil-l'ar t;

The carriage I is actuated for movement vertical and horizontaldi rection's' by bars mane I33, respectively; 1 The su porting barsfor the carriage have rollers I stand I35, respectively, on their ends, which ride-oniiidividually associated tracks I36 and I3I,which areprovided inthe base or the receiving-instrument.- Asthe rods I32 and I33 ar'e moved, the-pen carriage I3I] will be moved l inan direction underneath the chart I3 8. If rod I32-only is moved, asupen cperadencr me: tor I26; the carriage- I3I] will be moved in a vertical direction, only. Similarly, if rod I33 only is moved, as upon-operation of motor I26, the carriage I30, will be moved in a horizontal direction only. If rods I32 and I33 are, simultaneously moved, the pen carriage I30 will be moved at an angle to both sides" of the instrument. Movement is imparted to the rod I32 as previously stated. by the motor I26, whichhas a drive pulley I-3'9 attached to it; Thispulley drivesa cable I40 that passesover suitable guide pulleys at opposite ends or the track I36 and which is fastened to the roller 34. In alike manner motor I 23 is utilized to impart movement to the rod I33, and to this end motor I26 has a drive pulley I4I fastened to its shaft. As the pulley IN is rotated, it drives a cable I42 that passes around suitable guide pulleys at the ends of track I31 and which is attached to the roller I35.

In consequence, as the motors I26 and I26 are energized for rotation in one direction or the other in response to the reception of oscillating signals from the transmitter, the pen I3Ifwill be adjusted relative to the chart I38 to produce an exact reproduction of the movement imparted to the writing stylus or pencil at the transmitter relative to the plotting board 4. As previously noted, upon movement of the writing stylus or pencil into engagement with the plotting board at the transmitter, the switch 7 is closed and effects energization of the solenoid F6 for lifting the recording pen up into engagement with the under side of the recording chart I38. In consequence the recording pen I3I is operative to trace the path of its movement on the chart I38 only when the writing pencil or stylus at the transmitter is in contact with the plotting board.

This arrangement in which the recording mechanism is located entirely beneath the chart, and therefore leaves the top of the chart free for inspection and other purposes, constitutes no part of the present invention but is the invention of Barnes, Johnson and Wills and is disclosed and claimed in application Ser. No. 620,828, filed by them on October 6, 1945'.

Subject matter disclosed in this application and not claimed herein, and not claimed in the copending applications to which reference has been made, is disclosed and is being claimed in an application Serial Number 620,830, and an application Serial Number 620,832, both of which are being filed concurrently herewith by R. F. Wild.

While, in accordance with the provisions of the statutes, I have illustrated and described the best form of my invention now known to me, it will be apparent to those skilled in the art that changes may be made in the form of the apparatus disclosed without departing from the spirit of my invention as set forth in the appended claims, and that certain features of my invention may sometimes be used to advantage without a corresponding use of other features.

is tracing: new described my invehtih, what I claim as new and desire to secure by Letters Pat Means tov re the tendency resistortioii ef the simple ationsl'iip between frequency tunin capacity of the parallel resonant-err:

cent of an electrical cscrllationgenerator ha nig rallel' resorrant'tumng circuit tunable over a predetermlned'band. of frequencies and a'fe'ed back coil coupled to said parallelresonant circuit to maintain the-oscillator in an oscillating state; means to time said feedback coil to a value intermediate the upper endofsaid predetermined frequency. band and the lower-end of the corre sponding second harmonic frequency band.

v Means to minimize the tendency to distortion of the simplerelatioriship between frequency and tuning capacity. of the parallel resonantcir= cui't of? an electrical os'c'illatio'n generator having afeedbackcbil coupled to the parallel resonant tuning. circuit for sustaining the oscillator operatidn comprising; airosol l'lator provided with a parallel resonant tuning circuit tunable over a predetermined band of frequencies and a feedback coil coupled to said parallel resonant circult to maintain the oscillator in an oscillating state, and means to tune said feedback coil to a value intermediate the upper end of the second or a higher harmonic frequency band corresponding to said predetermined band of frequencies and the lower end of the next higher harmonic frequency band.

3. An oscillator operative in different conditions of adjustment to generate electrical oscillations of correspondingly different frequencies and comprising an electron discharge device having input elements and output elements, a parallel resonant circuit connected to the input elements of said device, the resonant frequency of said parallel resonant circuit being adjustable to vary the frequency of oscillation of said oscillator over a predetermined band of frequencies, an output circuit connected to the output elements of said device, said output circuit including a coil coupled to said parallel resonant circuit for regenerative operation of said device, and means to tune said coil to a frequency value which is approximately midway between the upper end of said predetermined band of frequencies and the lower end of the corresponding second harmonic frequency band.

4. An oscillator operative in different conditions of adjustment to generate electrical oscillations of correspondingly different frequencies and comprising an electron discharge device having input elements and output elements, a parallel resonant circuit connected to the input elements of said device, the resonant frequency of said parallel resonant circuit being adjustable to vary the frequency of oscillation of said oscillator over a predetermined band of frequencies, an output circuit connected to the output elements of said device, said output circuit including a coil coupled to said parallel resonant circuit for regenerative operation of said device, and means to time said coil to a frequency value which is approximately midway between the upper end of the second or a higher harmonic frequency band corresponding to said predetermined band of frequencies and the lower end of the next higher harmonic frequency band.

5. An oscillator operative in different conditions of adjustment to generate electrical oscillations of correspondingly different frequencies and comprising an electron discharge device having input elements and output elements, a tunable electric circuit path connected to the input elements of said device, the resonant frequency of said circuit path being adjustable to vary the frequency of oscillation of said oscillator over a predetermined band of frequencies, an electric circuit path connected to the output elements of said device and coupled to said first mentioned electric circuit path for regenerative operation of said device, said second mentioned electric circuit path being turned to a frequency value which is intermediate the upper end of said predetermined band of frequencies and the lower end of the corresponding second harmonic frequency band. 7

a 6. An oscillator operative in different conditions of adjustment to generate electrical oscillations of correspondingly different frequencies and comprising an electron discharge device having input elements and output elements, a tunable circuit path connected to the input elements of said device, the resonant frequency of said cir- REFERENCES CITED The following references are of record in the file of this patent: V a

UNITED STATES PATENTS Number Name a 7 Date 2,165,468 Farrington July 11, 1939 2,274,638 Rosene Mar. 3, 194 2,382,954 Beaudoin Aug. 21, 1945 2,415,718 Wilson et al. Feb. 11, 1947 2,446,392 Rey Aug. 3, 1948 

